1. Field of the Invention
The present invention relates to a compound semiconductor light emitting device, and in particular to a compound semiconductor light emitting device which can be suitably applied to an InP-semiconductor laser having a BH structure (Buried Hetero-structure); and a process for producing the same.
2. Description of the Related Art
Hitherto, a device having the following structure has been known as an InP-semiconductor laser having a BH structure. This device will be described referring to FIG. 4. FIG. 4 is a schematic light-emitting end face of an InP-BH structure type semiconductor laser in the prior art.
This device has an InGaAsP active layer 103 in a stripe form over an n-type InP substrate 101. On the upper and lower surfaces of the active layer 103, InGaAsP guide layers 105 are formed which have such a composition that the guide layers 105 have larger band gaps than the active layer 103. On the n-type substrate portions 101a at both sides of the active layer 103, there are formed a p-type InP sub-layer 107 having a carrier concentration of 5xc3x971017 cmxe2x88x923, and an n-type InP sub-layer 109 having a carrier concentration of 1xc3x971018 cmxe2x88x923, in this order from the lower to the upper. A p-type InP cladding layer 111 having a carrier concentration of 1xc3x971018 cmxe2x88x923 is formed over/on the active layer 103 and the n-type InP sub-layer 109. On the p-type InP cladding layer 111, a p-type InGaAs contact layer 113 is deposited. Electrodes 115 and 117 are formed on the upper surface of the p-type InGaAs contact layer 113 and the lower surface of the n-type InP substrate 101, respectivly.
In this semiconductor laser, a current path narrowing layer, that is, a current blocking layer composed of the p-type sub-layer 107 and the n-type sub-layer 109 is formed at both sides of the active layer 103. The p-type cladding layer 111 over the active layer 103, the n-type sub-layer 109, the p-type sub-layer 107 and the n-type substrate 101 constitute a pnpn structure. By this structure, the current injected to the device does not flow into other than the active layer 103.
In the p-type cladding layer 111 on/over the n-type sub-layer 109 and the active layer 103 in such a conventional semiconductor laser, its carrier concentration is raised to 1xc3x971018 cmxe2x88x923, in order to lower the resistance of the semiconductor laser. However, when the p-type cladding layer 111 is formed, Zn, which is a p-type dopant and may be introduced as DMZn: dimethylzinc (Zn(CH3)2), is diffused to the n-type sub-layer 109. As a result, in the n-type sub-layer 109 holes are generated. The holes and electrons, which are n-type carriers, are combined and extinguished so that the number of the n-type carriers in the n-type sub-layer 109 is reduced. Therefore, the function as the n-type of the n-type sub-layer 109 is deteriorated. Namely, carriers are canceled out. Thus, the performance as the current blocking layer, that is, the performance of injecting currents efficiently into the active layer 103 is deteriorated, resulting in a problem that the light emission efficiency of the semiconductor laser falls.
An object of the present invention is to provide a compound semiconductor light emitting device which makes it possible to keep the effect of confining carriers into an active layer and improve light emission efficiency. Another object of the present invention is to provide a process for producing a compound semiconductor light emitting device.
Therefore, the compound semiconductor light emitting device of the present invention comprises an active layer disposed on/over a first conductive type substrate; a second conductive type sub-layer and a first conductive type sub-layer, in this order from the lower to the upper, disposed on/over the first conductive type substrate and at both sides of the active layer; a second conductive type cladding layer disposed on/over the active layer and the first conductive type sub-layer; a second conductive type contact layer disposed on/over the second conductive type cladding layer; and a second conductive type diffusion barrier layer disposed between the first conductive type sub-layer and the second conductive type cladding layer.
The second conductive type sub-layer and the first conductive type sub-layer form a current blocking layer, and, accordingly, have the function of injecting a current efficiently into the buried active layer. Therefore, each of the sub-layers may also be called as a current block layer. The second conductive type diffusion barrier layer is disposed between the first conductive type sub-layer and the second conductive cladding layer. Therefore, when the light emitting device of the present invention is produced, the diffusion of the second conductive type dopant from the second conductive type cladding layer can be confined into the second conductive type diffusion barrier layer. For this reason, the second conductive type dopant is not incorporated into the first conductive sub-layer. Thus, the first conductive type carrier in the first conductive type sub-layer does not become extinct, so that its carrier concentration does not fall. Thus, the first and second conductive type sub-layers can cooperate to keep the function as the current blocking layer, and consequently the efficiency of injecting the current into the active layer can be improved, as compared with the prior art.
Preferably, each of the first conductive type substrate, the second conductive type sub-layer, the first conductive type sub-layer, the second conductive type cladding layer and the second conductive type diffusion barrier layer may be made of InP; and each of the active layer and the second conductive type contact layer may be made of InGaAs or InGaAsP.
When the semiconductor compound light emitting device is made of the aforementioned materials, Zn (zinc) is used as the second conductive type dopant for forming any second conductive type layer. If the second conductive type cladding layer contacts the first conductive type sub-layer, it is feared that Zn is diffused from the second conductive type cladding layer to the first conductive type sub-layer during the formation of the second conductive type cladding layer. Therefore, if the second conductive type diffusion barrier layer is beforehand formed between the second conductive type cladding layer and the first conductive type sub-layer, the diffusion barrier layer can take therein Zn. Accordingly, the diffusion of Zn to the first conductive type sub-layer can be restrained. Thus, the dopant concentration in the first conductive type sub-layer is not reduced during the formation of the device, so as to result in the value as designed. As a result, the carrier concentration in the first conductive type sub-layer also results in the value as designed.
Preferably, the second conductive type diffusion barrier layer may be a layer formed as follows. Namely, this layer is firstly formed as a preparatory (or provisional) layer having a lower carrier concentration than the carrier concentration in the second conductive type cladding layer. In the subsequent steps of forming the second conductive type cladding layer, the second conductive type dopant is diffused from the second conductive type cladding layer to the preparatory layer. By this diffusion, the preparatory layer is finally turned into a layer having the same or substantially the same carrier concentration as in the second conductive type cladding layer.
According to the above, the second conductive type diffusion barrier layer becomes a layer substantially functioning as a part of the second conductive type cladding layer in the compound semiconductor light emitting device. As a result, the first conductive type sub-layer can keep the effect as the current blocking layer. Since the second conductive type diffusion barrier layer becomes a part of the second conductive type cladding layer, no bad effect is produced on the compound semiconductor laser.
Furthermore, a process for producing a compound semiconductor light emitting device comprises: the first crystal growth step of epitaxially growing an InGaAs active layer and a second conductive type, first InP cladding layer in turn on/over a first conductive type substrate; the step of disposing an etching mask in a stripe form on/over the second conductive type, first InP cladding layer, and etching an area uncovered with the etching mask to a depth reaching the first conductive type substrate; the second crystal growth step of epitaxially growing a second conductive type InP sub-layer, a first conductive type InP sub-layer, and a second conductive type InP diffusion barrier layer in turn on/over an uncovered area of the first conductive type substrate which is uncovered with the etching mask; the step of removing off the etching mask; and the third crystal growth step of epitaxially growing a second conductive type, second InP cladding layer and a second conductive type InGaAs contact layer in turn on/over uncovered upper surfaces of the second conductive type, first InP cladding layer and the second conductive type diffusion barrier layer.
The second conductive type diffusion barrier layer can be formed in the second crystal growth step, wherein the second conductive type and first conductive type InP sub-layers are formed. Thus, increase in steps is unnecessary for forming the second conductive type diffusion barrier layer, so that the device can be easily produced.
Preferably, a dopant for the first conductive type may be Si2H6, and a dopant for the second conductive type may be zinc (Zn).
Preferably, the second conductive type InP diffusion barrier layer may be formed as follows. Firstly there is formed a preparatory (or provisional) layer having a lower carrier concentration than the carrier concentration in the second conductive type, first InP cladding layer in the first crystal growth step. After that, the second conductive type dopant is diffused from the second conductive type, second InP cladding layer to the preparatory layer, when the second conductive type, second InP cladding layer is grown, whereby the carrier concentration in the preparatory layer is made the same or substantially the same as in the second conductive type, second InP cladding layer.
According to the above, the second conductive type InP diffusion barrier layer itself substantially becomes a part of the second conductive type InP cladding layer after the second conductive type InP diffusion barrier layer itself causes restraint of the diffusion of the second conductive type dopant to the first conductive type InP sub-layer. Accordingly, in the compound semiconductor light emitting device having substantially the same structure as such devices in the prior art, the effect of confining carriers into the active layer can be improved.
Preferably, the second conductive type InP sub-layer, the first conductive type InP sub-layer, the second conductive InP cladding layer and the preparatory layer may be formed so that the carrier concentrations in these layers will be 5xc3x971017 cmxe2x88x923, 1xc3x971018 cmxe2x88x923, 1xc3x971018 cm31 3 and from 5xc3x971017 to 7xc3x971017 cm31 3 (both inclusive), respectively.
More preferably, the preparatory layer may be formed so that the carrier concentration therein will be 5xc3x971017 cmxe2x88x923.
Furthermore, preferably, when the second conductive type InP cladding layer may be formed so that the carrier concentration therein will be 1xc3x971018 cmxe2x88x923, the second conductive type InP diffusion barrier layer is formed so that the carrier concentration therein will be 1xc3x971018 cmxe2x88x923 or about 1xc3x971018 cmxe2x88x923.
According to the above, the second conductive type dopant is diffused into the preparatory layer, so that the carrier concentration in the preparatory layer rises. Finally, the carrier concentration in the second conductive type InP diffusion barrier layer becomes substantially the same as that in the second conductive type InP cladding layer. At this time, the diffusion of the second conductive type dopant is finished, thereby restraining the diffusion of the second conductive type dopant into the first conductive type InP sub-layer positioned below the preparatory layer. Furthermore, the above makes it possible to make the second conductive type InP diffusion barrier layer into a layer functioning as a part of the second conductive type InP cladding layer in the compound semiconductor light emitting device.
It is preferred that the first conductive type may be made into an n-type and the second conductive type may be made into a p-type.
It is also preferred that the etching mask may be made of a SiO2 or SiN film.
It is also preferred that the aforementioned film may be formed by a CVD process.
Furthermore, it is preferred that the crystal growth in the first, second and third crystal growth steps may be carried out by using a vapor phase or liquid phase growth process.
The vapor phase process may be, for example, an MOVPE (Metal Organic Vapor-Phase Epitaxy) process.